1. Field of the Disclosure
The present invention relates to the field of depth measurement in a wellbore. In particular, the present invention relates to measuring distances down-hole by observing a phase shift associated with a signal traversing a signal conductor associated with a wellbore tubular.
2. Description of the Related Art
Oilfield wellbores are drilled by rotating a drill bit conveyed into the wellbore by a drill string. The drill string includes a drill pipe (tubing) that has at its bottom end a drilling assembly (also referred to as the “bottomhole assembly” or “BHA”) that carries the drill bit for drilling the wellbore. The drill pipe is made of jointed pipes. Alternatively, coiled tubing may be utilized to carry the drilling of assembly. The drilling assembly also includes a variety of sensors for taking measurements of a variety of drilling, formation and BHA parameters. Exemplary tools include measurement while drilling (MWD) tools. As is known, it is often necessary to know the drill bit depth, from which is usually derived the hole depth and the tool depth of MWD tools. Conventional depth estimates are generally based on surface measurements, either using a pipe stand length or a pipe travel measurement (from the draw works for example) or both and a periodic survey points (surveys). The pipe travel is generally referred to as rate of penetration, ROP, and is generally provided in m/hr or ft/hr. To the surface measurements, various corrections are made for pipe stretch, temperature, non-linearities in the pipe travel measurements, etc.
While conventional depth measurements systems have been adequate in certain applications, there is an ever present need for more cost-effective and accurate depth measurement systems.